Femtosecond pulsed laser excitation of coherent and squeezed phonon fields

Femtosecond laser pulses can be used to create and detect temporally coherent phonon fields in solids through a time-resolved stimulated Raman scattering process. A strong laser pulse, acting as an impulsive force, pumps a fundamental excitation in the material, while a weaker pulse probes the dynamics of the excitation. A description of femtosecond stimulated Raman scattering as observed with this pump-probe technique is presented. Experimental results are summarized for the materials lanthanum aluminate (LaAlO₃ ), antimony (Sb), potassium tantalate (KTaO₃) and strontium titanate (SrTiO₃), in terms of the new effects and extensions to the basic theory they present. With LaAlO₃, we identify the differences between scattering from a coherent phonon field at the surface, or interface, of the sample and scattering in the bulk. In Sb, we investigate resonance effects in the excitation and detection of coherent phonon fields and how the imaginary component of the Raman susceptibility enters into calculations. And with KTaO₃ and SrTiO₃, we show how a coherent two-phonon Raman scattering process excites the sample into a state in which the uncertainty in the atomic positions of the lattice atoms oscillates as a function of time. When the sample is cold enough that quantum fluctuations dominate, it is shown that we create an oscillatory squeezed state in which the variance in the ion displacements falls below the standard quantum limit for part of the cycle.